1. Field of the Invention
The present invention relates to the field of telecommunications, and more particularly, a method and apparatus for controlling the power of a transmitted signal.
2. Description of Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multi-carrier modulation having inherent robustness against multipath effect. For example, IEEE 802.11a specifies the Physical Layer Entry for an OFDM system that provides a wireless Local Area Network (LAN) with data payload communication capabilities from 6 to 54 Mbits/sec in the Unlicensed National Information Infrastructure (U-NII) frequency band. The system uses 52 sub-carriers which are independently modulated by using Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16-QAM) or 64-Quadrature Amplitude Modulation (64-QAM) associated with different coding rate for different data speed.
A major challenge for an OFDM-based communication system is the inherent high crest factor (peak-to-average ratio) of multi-carrier systems. Considerable output power back-off from the power amplifier (PA) saturation region will be needed to avoid distortion and spectral regrowth. The back-off for the power amplifier, however, reduces its efficiency. Because the peak transmitted power is usually constrained by regulatory limits, a large back-off of the power amplifier design to deal with the high crest factor has the effect of significantly reducing the average transmit power. The low average transmit power introduces several problems such as reducing radio coverage and making the transmitted signal more susceptible to interference.
So far, several crest factor reduction techniques have been proposed such as Reed-Muller codes, Golay sequences, subsets of block coding that avoid transmitting codewords with a large crest factor, and selective sub-carrier mapping to reduce the crest factor. However, as the number of sub-carriers increases, the coding rate slows and the coding process becomes more complicated (e.g. extensive computation, search, look-up tables). Unlike cellular/PCS systems that can afford costly power amplifiers, the power amplifier used in a wireless LAN needs to be simple and cheap. Clipping the OFDM signal is another way to reduce the crest factor. Clipping can be described as limiting the peak amplitude of an OFDM signal to the power amplifier input so that the undesirable effect of the amplifier non-linearity problem can be controlled. However, inadequate clipping introduces excessive out-of-band distortion.